During carbon steel manufacturing, large amounts of electric arc furnace (EAF) slag are generated. EAF slag, if properly treated and processed into aggregate, is an alternative source of high-quality material, which can substitute the use of natural aggregates in most demanding applications in the construction sector, mostly for wearing asphalt courses. In this screening process of high-quality aggregates, a side material with grain size 0/32 mm is also produced, which can be used as an aggregate for unbound layers in road construction. In this study, the environmental impacts of slag aggregate (fraction 0/32 mm) were evaluated in mixed natural/slag aggregates. Different mixtures of natural/slag aggregates were prepared from aged (28 days) and fresh slag, and their environmental impacts were evaluated using leaching tests. It was shown that among the elements, chromium (Cr) was leached from some mixed aggregates in quantities that exceeded the criterion for inert waste. The data from the present investigation revealed that mixed aggregates, prepared from aged slag (fraction 0/32 mm) and natural stone in the ratio 10/90, are environmentally acceptable and can be safely used in unbound materials for road construction.